This application is related to metallic surface finishing and, specifically, to chemical assisted lapping and polishing of metallic surface, e.g., gears, using aqueous based carriers.
Lapping and polishing are widely used precision finishing processes for many objects made of a variety of materials including glass, ceramic, plastic, semiconductor metals, metals and their alloys (Loose Abrasive Processes, Chapter 13, in Tribology of Abrasive Machining Processes by I. D. Marinescu et al, Noyes, 2004). The objects suitable for the two processes are also diverse in terms of their sizes, shapes, and functions.
Gears are fundamental and essential components of transmission equipment, which are used for construction of both military and civilian machinery. Most high performance gears today are still made with steel and are manufactured by first machining the steel into a blank followed by cutting to form teeth. The rough gears are then heat treated and finished with varying methods depending on requirement of the gear specifications. Surface smoothness is an important parameter in evaluating gear performance. Generally speaking, the finely finished gear teeth could reduce the amount of friction between the contact surfaces, minimize pitting, scuffing, and wear, thus prolonging working life of gears. In addition, it also reduces the harmonic noise when gears are in use. Improvement in gear manufacturing technology has always been an interest for many parties (Manufacturing Technology Research Needs of the Gear Industry, A Manufacturing Technology Information Analysis Center Report by M. A. H. Howes, December 1987).
U.S. Pat. No. 6,732,606 discusses that an optimally finished gear should have a surface roughness between approximately 5 micro-inches Ra to 10 micro-inches Ra. The reduced surface roughness, when located at the gear teeth, can reduce the maximum contact stress by more than fifty percent. The subsurface shear stress can be reduced by approximately thirty percent. Various finishing methods to achieve the stated Ra are provided, including examples such as electrochemical polishing and physicochemical polishing.
Even though many methods are available for gear finishing (Finishing Methods, in Surface Engineering, ASM Handbook, Vol. 5, page 79-164, ASM International, Materials Park, Ohio, 1994), lapping is by far the most well established process for finishing the tooth surfaces of a pair of gears, which are used in mesh with each other, especially for bevel gears. During lapping, loose abrasive slurry, referred as lapping compound, is introduced to the meshing tooth surface of two gears under load conditions, usually through nozzles near the meshing point of the teeth. The process is usually involved using specialized and expensive lapping machinery. U.S. Pat. Nos. 6,217,421, 6,120,355, 5,538,462, 5,299,390 and US Patent Application Publication Nos. US-2001/0030152, US-2006/0111018, and US-2006/0185184 represent recent innovations in designing of the lapping equipment.
The lapping compound typically includes abrasive grains or grits suspended in a liquid carrier. The abrasive grains or grits provide the action force of abrading under pressure and the liquid carrier provides a vehicle that suspends the abrasives and lubricates or cushions the interface to minimize metal to metal contact, reducing potential surface damage and heat formation. Typically, machines using traditional lapping compounds must be operated under high pressures for optimal production efficiency. However, under such conditions, the cutting edges of the grains and grits are worn out easily, rendering frequent change over of lapping compound for disposal.
Historically, oil-based carriers are used in the formulations for the lapping compounds. Suitable liquids include kerosene, diesel fuel, mineral oil, seal oil, spindle oil, and vegetable oil. In certain cases, heavy oils, greases and animal fats may also be used in the composition of the carrier. To improve the oil based carrier, a variety of oil soluble chemicals may be added. For example, U.S. Pat. Nos. 4,046,524 and 4,770,672 refer to oil-based lapping compositions. Use of an oil based carrier as a vehicle for lapping operations suffers from several shortcomings including the need for heavy cleaning of processed parts, expensive waste disposal of used lapping compound, and potential slip and fall hazard for lapping operators. In addition, the ever-increasing crude price also makes the oil based lapping process less competitive.
Water based lapping compounds have also been proposed in the art. See, for example, U.S. Pat. Nos. 4,038,048 and 5,855,633 in which the aqueous carrier, like oil based carrier, strictly serves as a vehicle to suspend loose abrasives and provide certain degrees of lubricity.
Chemical assisted mechanical finishing has long been known and gained wide acceptance in the semiconductor industry. Chemical mechanical planarization (CMP) is now an integrated process in semiconductor chip manufacturing and continues to be a subject of active research, seeking improvement on all aspects of the technology. See, US Patent Application Publication Nos. US-2006/0131275, US-2005/0037936, and US-2004/0134873 and U.S. Pat. Nos. 7,029,373, 6,838,149, 6,569,350, 6,416,685, and 6,236,542. The technology usually uses strong and aggressive chemicals to help remove unwanted metals on the chip surface. The common chemicals used by these processes include hydrofluoric, hydrochloric, nitric acids, hydrogen peroxide and other oxidants. Due to the aggressive nature of the chemicals, sophisticated process control is incorporated in the technology to avoid over-finishing.
In the metalworking industry, chemical assisted finishing also utilizes strong acids for metal surface finishing, which tends to etch pits and scratches deeper on the processed metal parts, making the process difficult to control, even though processes using mild acids such as in U.S. Pat. No. 3,979,858 might alleviate the problem to certain degrees. Over all, careful process control is required.
To avoid the potential problem, U.S. Pat. No. 3,071,456 refers to the use of “surface conversion agents”, which form a friable layer on metal surface, with the metal removal on the surface being not only accelerated, but also selective, avoiding the phenomenon of deepening pits and scratches on the metal surface normally associated with conventional acid system. The “surface conversion agents” include various phosphoric acids and their salts, nitro compounds, organic acids such as citric and oxalic acids. Also discussed is barrel finishing, a form of vibratory mass finishing, using abrasive media. However, the oxalic and phosphorus based conversion coatings suffer from operation inefficiency, partly due to the limited solubility of these reagents in water, especially the salts of the same.
Still other proposals for metalworking or finishing compositions and processes are provided by U.S. Pat. Nos. 4,181,540; 4,491,500; 4,705,594; 4,818,333; 3,071,456; 4,906,327 (RE 34,272); 5,051,141; 5,158,623; 5,158,629; 7,005,080; and 4,818,333 and US Patent Application Publication No. US-2005/0164610. Other documents also address applications of the conversion coating assisted finishing in production of bearings and gears, such as U.S. Pat. Nos. 5,503,481 and 5,873,770. Chemically accelerated vibratory gear finishing using high density, non-abrasive ceramic media is also known. Publications related to this subject include G. Sroka and L. Winkelmann titled as “Superfinishing Gears” in July/August 2005 issue of Gear Technology, pages 30-32 and US Patent Application Publication No. US-2005/0014597. A related process is also discussed in U.S. Pat. No. 6,217,415.
To summarize, the conversion coating assisted chemical finishing process is normally practiced using vibratory mass finishing equipment. The media used in the vibratory equipment could be either abrasive or non-abrasive. In practice, the conversion coating assisted finishing processes in the art suffer from operation inefficiency. Generally, the processes need a minimum of several hours to achieve the desired smoothness of metal surface, as discussed above. This time frame is unacceptable for lapping operations, since it involves expensive lapping machinery and usually processes one item at a time. For example, commercial gear lapping only allots a few minutes for a pair of gears.
There remains a need in the art for alternate and improved compositions and methods for lapping gears efficiently.